Hello Hartmut,
They look partly similar to the Icons I have in <install_dir>/lazarus/images/states/ of my Lazarus 2.0.10 installation.
Yes, but these icons are available in the sizes 16x16, 24x24, and 32x32.
If you don't want your program to be high-DPI (
https://wiki.freepascal.org/High_DPI), then you can just use a 32x32 icon for your dialog (or any other size).
For a High-DPI application there should typically be three icon sizes. For dialog windows, Windows uses 32x32 icons at the default resolution of 96 DPI.
So you still need a 48x48 and a 64x64 icon. As said, only if the application should be High-DPI capable.
However, applications that are not High-DPI capable, on higher resolution screens, usually lead to an unsatisfactory result.
Greetings Roland